Postal workers are to vote on a national strike. Photographer: Kirsty Wigglesworth

Royal Mail's management and union clashed today over the decision to ballot workers over industrial action.

The Communication Workers Union blamed Royal Mail for provoking the national ballot, accusing it of failing to consult over new working practices. The management called the ballot unjustified and claimed that the union had walked away from negotiations.

The result of the ballot will be announced early next month and a series of strikes, which would halt mail deliveries across the country, could start soon afterwards.

The CWU staged a number of strikes at regional sorting offices during the summer, but had stopped short of a national ballot.

Royal Mail is losing badly needed business because of the disruption, which the company estimates is affecting one in eight letters and parcels.

Last month, an estimated 100m extra bulk mail items were handled by rivals as a result of the regional stoppages, according to Post-Switch, a postal analyst and advisory service. It said Royal Mail lost more than twice as much business as normal, as companies turned to other operators.

The dispute makes the predicament of Royal Mail, which is already technically insolvent because of an estimated £10bn pension deficit, even more dire.

Its management says Royal Mail is already handling 10% less mail each year as the use of email and the internet grows. It has warned that industrial action could hasten the end of the one-price universal mail service, as every 1% of decline in business costs an estimated £70m in lost revenues.

Dave Ward, the union deputy general secretary, said on BBC Radio 4's Today programme that the company was putting staff under "intolerable pressure" to make efficiencies, although he conceded that increasing automation would bring redundancies.

But Mark Higson, the company's managing director, accused the union of reneging on a 2007 agreement on pay and modernisation. He said it "beggared belief" that Ward admitted that the disruption was hurting customers while calling "a national strike ballot to step up the damage they are already inflicting on customers big and small".

Later, Higson apologised to customers and said Royal Mail's changes were essential to respond to the market.

The 2007 agreement was only reached after the last round of national postal strikes. Employment experts believe that unions are trying to delay modernisation because it will result in more redundancies, which could become compulsory if the company cannot reduce numbers on a voluntary basis.

Royal Mail has shed 50,000 posts since 2002 and has promised the CWU it will not make compulsory redundancies. But as the volume of mail drops and pressure to cut costs rises, sticking to this pledge will become increasingly difficult.